Treatment with the kinase inhibitors gefitinib (Iressa) and erlotinib (Tarceva) has shown promising results for patients with lung cancer, the leading cause of cancer death in the United States. Activating mutations in the epidermal growth factor receptor tyrosine kinase gene, EGFR, occur frequently in lung adenocarcinoma and are associated with response to these kinase inhibitors. During the past year, we and others have identified several new classes of EGFR mutation in human tumors, as described in the "Preliminary Data" section. 1) A secondary kinase domain mutation, T790M, confers resistance to gefitinib and erlotinib in the setting of an activating, inhibitor-sensitive EGFR mutation. 2) One class of lung-cancer derived activating EGFR mutations, insertions within exon 20, are resistant to erlotinib and gefitinib. 3) Somatic mutations of the EGFR extracellular domain are found in glioblastoma. I now propose to conduct further studies to explore the mechanism of EGFR-dependent transformation and the mechanisms for resistance to EGFR inhibitors for tumor-derived EGFR mutations. The long-term goal of this work is to improve treatment of lung adenocarcinoma, glioblastoma, and other EGFR-dependent tumors. Specifically, I propose the following aims, tied to the recently discovered classes of EGFR mutants. Specific Aim 1. Test the inhibition of EGFR-dependent cell growth by specific small molecule EGFR inhibitors for a panel of transforming EGFR mutations. Specific Aim 2. Analyze the structural and biochemical determinants of EGFR-dependent transformation. Specific Aim 3. Identify secondary mutations in activated EGFR that cause resistance to EGFR inhibitors such as gefitinib. Specific Aim 4. Determine whether cancer-derived point mutations in the extracellular domain of EGFR are transforming and whether these mutants can be inhibited by small molecule kinase inhibitors.